1. Field of the Invention
The present invention relates to a transformer and a communication terminal device that includes the transformer.
2. Description of the Related Art
As wireless communication devices such as cellular phone terminals become smaller, there is a tendency that antennas become smaller and impedance thereof becomes lower. To match an antenna having an impedance that is significantly lower than that of a power feeding circuit, with the power feeding circuit by using a reactance element, that is, when the impedance conversion ratio is great, the frequency band to match becomes narrow.
Meanwhile, when a single antenna is used to handle a plurality of communication systems, such as a low-band (e.g., 800 MHz frequency band) communication system and a high-band (e.g., 2 GHz frequency band) communication system, a basic resonant mode and a high-order resonant mode of one radiating element are used. However, the impedance of the radiating element is different in accordance with frequency. When a matching circuit for matching in one frequency band is provided, this matching circuit is incapable of matching in the other frequency band.
In order to solve the above-described problem, there is proposed an impedance converting circuit in which a transformer circuit is used as a matching circuit, as described in Japanese Patent No. 4761009.
In general, when the primary coil and the secondary coil of a transformer have an identical shape, the closer they are arranged to each other, the higher the coupling coefficient between the primary coil and the secondary coil. When a transformer is configured in a multilayer body including a plurality of substrate layers that are laminated, the coupling coefficient becomes higher if loop-shaped coil conductors are positioned closely to each other in a direction in which the substrate layers are laminated.
To connect one coil conductor to another coil conductor or to an external terminal, it is necessary to form wiring outside the areas where the coil conductors are formed. Because of this wiring, which contributes nothing to the coupling between the primary coil and the secondary coil, the coupling coefficient between the primary coil and the secondary coil deteriorates.
To configure an impedance converting circuit with a transformer configuration or a multilayer coil such as a common-mode choke coil, the above-mentioned wiring outside the areas where the coil conductors are formed becomes an obstacle to size reduction. That is, some distance for maintaining an insulated state is necessary between an interlayer connection conductor that connects layers and a coil conductor. As a result, it is necessary to reduce the size of the areas where the coil conductors are formed. On the other hand, as the areas where the coil conductors are formed become smaller, the inductance obtained per layer becomes smaller. To obtain a desired inductance, it is necessary to increase the number of layers where coil conductors are formed. This results in not only the deterioration of the coupling coefficient, but also the deterioration of the Q value of the coils. To dispense with wiring in the multilayer body, wiring may be provided using side surface electrodes on the multilayer body. However, the degree of freedom in changing the positions of side surface electrodes on the multilayer body is very low since their positions are determined in advance according to sizes or usage.
To increase the coupling coefficient between the primary coil and the secondary coil, it is important that each of the primary coil and the secondary coil have coil conductors having an identical shape. FIGS. 8A and 8B are plan views of two substrate layers 11 and 12 on which coil conductors are formed. FIGS. 8C and 8D are plan views of the substrate layers 11 and 12 laminated together. FIG. 8A illustrates an example where a coil conductor equivalent to substantially one turn is formed on each of two substrate layers, and a power feeding terminal P1, an antenna terminal P2, and a ground terminal P3 are formed at one end portion of the multilayer body. FIG. 8B illustrates an example where the above-mentioned terminals are formed at top/bottom and right/left portions of the multilayer body. In both cases, interlayer connection is established by an interlayer connection conductor V at a certain position of each coil conductor. Although the configuration illustrated in FIGS. 8A and 8C obtains a high coupling coefficient, the configuration has little degree of freedom with regard to positions for forming side surface electrodes. The configuration illustrated in FIGS. 8B and 8D does not obtain a high coupling coefficient.
To have a position for forming each terminal as a defined position, it is effective to form coil conductors over a plurality of layers. For example, as illustrated in FIG. 9, coil conductors are formed on a plurality of substrate layers 11 to 15, and interlayer connection conductors V1 to V4 are formed at certain positions. As a result, a coil equivalent to one or more turns can be formed, and terminals can be arranged at end portions, facing each other, of the multilayer body. However, this configuration requires many substrate layers, and it becomes difficult to obtain a necessary (small) inductance value.